doc: add section stating that very stale PRs should be closed

doc/contributing/pull-requests.md

@@ -446,6 +446,17 @@ credit for the work they started (either by preserving their name and email
 address) in the commit log, or by using an `Author:` meta-data tag in the
 commit.
 
+If a pull request has been inactive for more than six months, it is unlikely
+to be merged. The contributor may no longer be engaged, may have lost context
+on their changes, the Node.js codebase may have evolved to the point where
+resuming the effort is impractical, or all of the above.
+To keep the project well organized and more maintainable, such stale pull
+requests should generally be closed unless they provide clear ongoing value
+or have a well defined path forward. When closing a pull request, always
+leave a polite comment explaining the reason and encouraging the contributor
+to restart the effort and submit a new pull request if the changes are still
+relevant and applicable.
+
 ### Approving a change
 
 Any Node.js core collaborator (any GitHub user with commit rights in the